The effects of
cysteine and
reduced glutathione (GSH) on the genotoxicity of
o-phenylphenol (
OPP) and its metabolites,
phenylhydroquinone (PHQ) and
phenylbenzoquinone (PBQ), were examined using the frequency of sister-chromatid exchanges (SCEs) and
chromosome aberrations in CHO-K1 cells as parameters. Cytotoxic (cell-progression delay) and cytogenetic effects induced by a 3-h treatment with
OPP, PHQ (100 micrograms/ml) or PBQ (50 micrograms/ml) with S9 mix after a 27-h expression time were inhibited by
cysteine or GSH (3-10 mM). Materials corresponding to the
cysteine or GSH adducts were found by HPLC in each incubation mixture. In the culture without S9 mix, PHQ and PBQ showed severe cytotoxicity since no metaphases could be obtained at doses over 25 and 5 micrograms/ml, respectively, and the
sulfhydryl compounds inhibited the toxicity by the formation of adducts with PBQ and by inhibiting the formation of PBQ in the case of PHQ. With PHQ, the
sulfhydryl compounds appeared to inhibit autooxidation. However, the
sulfhydryl compounds did not inhibit the cytotoxic and cytogenetic effects caused by
OPP in the cell mixture without S9 mix, but on the contrary intensified them. No adduct formation was detected in the incubation
solution. On the basis of these results, it is considered that electrophilic
quinone (PBQ) and/or semiquinone (phenylsemiquinone, PSQ) radicals, capable of binding to nucleophilic small molecules (such as
cysteine and GSH) or (
biological) macromolecules, are produced from metabolite PHQ in metabolic oxidation of
OPP, and induce cyto- and geno-toxic effects in the cells. The cyto- and geno-toxic effects of
OPP itself to the cells are clearly independent of any electrophilic radical reaction.